After trying different kinds of scrubbers, and other ways to reduce nitrates and phosphates I finally after trial and error for my tank did something that paid off. I added more indirect light to my waterfall scrubber using a red/blue spotlight I got on ebay - 20 Watt. This has caused my waterfall scrubber to finally take off - now I am getting lots of thick green algae where before I got a fairly thin layer over the screen. My nitrates are 0, phosphates look to be close to 0 as well. Corals are happy. Point of this post is to emphasize that you need to have flow, the right amount of light [not too close, not too far away] for the amount you feed and so on. You also have to keep in mind that for your scrubber to overcome the algae in your display, you need to have the right kind and amount of light in the display vs. what you put on your scrubber so that the scrubber grows the algae. Sometimes it takes trial and error over time to figure out the best solution.
